[alsa-devel] OSS emulation and hardware configuration

[Takashi Iwai]

At Wed, 14 Sep 2011 12:08:57 -0400,
Sean McNamara wrote:
> 
> Hi,
> 
> On Wed, Sep 14, 2011 at 11:20 AM, Mike Crowe <drmikecrowe at gmail.com> wrote:
> >> Does the changing of the rate actually succeed?
> >>
> >
> > Surprisingly, that's hard to answer.  Normally, most of our audio is
> > at 48K (we control the sound files), so the device stays at that rate.
> >  However, a new library we added (for voip communications) is
> > hard-coded to 8K sample rate via OSS, and I'm now trying to work out
> > dual usage by our ALSA code and this library's OSS interface.
> >
> > Before I added these kernel printouts, I was convinced the system was
> > switching rates properly (because I added 8K rate support to the
> > hardware driver to get the voip library working).  However, my current
> > debug load shows the rate changing in the pcm_oss module, but I never
> > see that rate hit the hardware the way I expected.
> >
> > If it isn't hitting the hardware, I suppose it could be up/down
> > converted by ALSA, but I don't have any plugins installed.  I thought
> > I needed a plugin for ALSA to convert between the two rates, right?
> 
> Right, but if you're using the in-kernel OSS emulation, it bypasses
> the userspace ALSA-lib plug layer! You'll have to resample it in
> userspace within the OSS application, probably.

Kernel OSS emulation has a rate converter, too, when
CONFIG_SND_PCM_OSS_PLUGINS is enabled.
But no soft-mixing or other complex stuff.


Takashi